The present invention is related to micro electron field emitter devices and, in particular, to enhanced Spindt tip emitters that may include a sharpening feature, an increased depth of dielectric layers between metal layers without concomitant increase in tip to aperture distances, and pull-back of dielectric surfaces from the emitter tip.
The present invention relates to design and manufacture of field emitter tips. A brief discussion of field emission and the principles of design and operation of field emitter tips is therefore first provided in the following paragraphs, with reference to FIG. 1.
When a wire, filament, or rod of a metallic or semiconductor material is heated, electrons of the material may gain sufficient thermal energy to escape from the material into a vacuum surrounding the material. The electrons acquire sufficient thermal energy to overcome a potential energy barrier that physically constrains the electrons to quantum states localized within the material. The potential energy barrier that constrains electrons to a material can be significantly reduced by applying an electric field to the material. When the applied electric field is relatively strong, electrons may escape from the material by quantum mechanical tunneling through a lowered potential energy barrier. The greater the magnitude of the electrical field applied to the wire, filament, or rod, the greater the current density of emitted electrons perpendicular to the wire, filament, or rod. The magnitude of the electrical field is inversely related to the radius of curvature of the wire, filament, or rod.
FIG. 1 illustrates principles of design and operation of a field emitter tip. The field emitter tip 102 rises to a very sharp point 104 from a silicon-substrate cathode 106, or electron source. A localized electric field is applied in the vicinity of the tip by a first anode 108, or electron sink, having a disk-shaped aperture 110 above and around the point 104 of the field emitter tip 102. A second cathode layer 112 is located above the first anode 108, also with a disk-shaped aperture 114 aligned directly above the disk-shaped aperture 110 of the first anode layer 108. This second cathode layer 112 acts as a lens, applying a repulsive electronic field to focus the emitted electrons into a narrow beam. The emitted electrons are accelerated towards a target anode 118, impacting in a small region 120 of the target anode defined by the direction and width of the emitted electron beam 116. Although FIG. 1 illustrates a single field emitter tip, field emitter tips are commonly micro-manufactured by microchip fabrication techniques as regular arrays, or grids, of field emitter tips.
Spindt tips are electron field emitter microdevices, such as the field emitter tip shown in FIG. 1, in which the conical emitter tip is deposited by sputter deposition of a suitable metal or metal alloy onto a substrate. The deposition is carried out following layering and patterning of the dielectric and metallic layers that form the extraction cathode layer and lensing anode layer (108 and 112 in FIG. 1).
Spindt tips are well known in the art, and techniques for fabricating Spindt tips have been developed by designers and manufacturers of field emission devices. However, current Spindt tip designs and fabrication techniques suffer from numerous recognized deficiencies. Current techniques lead to application of Spindt emitter tips relatively closely surrounded by a cylindrical well through the dielectric and metal layers perpendicular to the substrate on which the emitter tip is deposited. Undesirable electrostatic charges may build up on the dielectric surfaces of the well during Spindt tip operation. It is well known that the very fine points of field emitter tips may be contaminated with absorbed contaminants and/or deformed during usage, greatly effecting the current density of emitted electrons. Once fabricated, Spindt tips are notoriously difficult, or impossible, to sharpen and clean in order to restore optimal performance. Current fabrication techniques limit the width of dielectric layers separating metallic layers to approximately the height of the final Spindt tip, so that the point of the Spindt tip is positioned within or near the aperture of the electron extraction cathode, but because of the relatively strong electric fields employed to operate field emission devices, the maximum allowed width of the dielectric may be insufficient to completely prevent dielectric breakdown and shorts between positively and negatively charged metallic layers within the Spindt tip emission device. For these reasons, designers and manufacturers of Spindt tip field emitter tips have recognized the need for a design and manufacturing technique that avoids these recognized deficiencies.
One embodiment of the present invention is an enhanced electron field emitter Spindt tip with a built-in cleaning and sharpening feature, increased thickness of dielectric layers that increases the breakdown voltage threshold of the device, a greater distance between the field emitter tip and surrounding dielectric surfaces, and a method that allows for increased fabrication precision and that allows for economical and efficient addition of additional metallic layers that allow the direction of the electron beam emitted from the field emitter tip to be controlled. Additional fabrication precision is made possible by using two-layer dielectric bilayers within the device: a SiO2 sublayer and a Si3N4 surface layer that serves as a lateral oxide etch stop during etching of internal chambers. In the enhanced Spindt-tip device, the Si3N4 surface layer also coats the dielectric portions of the walls of the cylindrical well in which the Spindt tip is deposited, and is pulled back from close proximity to the Spindt tip between the metallic layers. Pulling back the Si3N4 surface layer prevents build-up of electrostatic charge during operation of the Spindt tip and allows for increasing thickness of the dielectric bilayer without, at the same time, increasing the distance between the point of the Spindt tip and the electron extraction anode aperture. A thin-film resistive heating layer is added to the surface of the substrate, between the base of the Spindt tip and the substrate surface. By passing current through the thin-film resistive heating element layer, the Spindt tip can be heated to high temperatures in order to both sharpen the tip and to remove contaminants adsorbed to the tip. Tip sharpening reduces the radius of the tip and correspondingly increases the current density of emitted electrons during operation. The method that represents one embodiment of the present invention for fabricating enhanced Spindt tips employs metal chemical-mechanical-planarization (xe2x80x9cCMPxe2x80x9d) in place of oxide CMP used in currently available methods to allow planarization of the metal layers and more precise control of the positioning of the point of the Spindt tip relative to the field extraction anode.